1. Field of the Invention
The present invention relates to a high-frequency wave measurement substrate used for measuring electrify characteristics of semiconductor elements, semiconductor element package or circuit boards which use a microstrip line in high frequencies such as microwaves and millimeter waves, more particularly to a wide-band low-loss high-frequency wave measurement substrate whose measurable frequency band is enhanced.
2. Description of the Related Art
For measurement and evaluation of electric characteristics of a semiconductor element, a semiconductor element package or circuit board in a high-frequency band such as a microwave or a millimeter wave, a wafer probe is used at the measuring instrument side, which comes in contact with a coplanar line to enable its highly accurate measurement. On the other hand, a microstrip line is usually used as a transmission line at an input/output part of a measurement object such as a fast digital or high-frequency circuit for radio communication apparatuses using high-frequency wave signals, a high-frequency semiconductor element, and a package for housing such a high-frequency semiconductor element. Consequently measurement of electric characteristics in a high-frequency wave using a wafer probe needs a line converter to cope with a connection between the coplanar line of the wafer probe and the microstrip line of the measurement object. The line converter is required to transmit high-frequency wave signals without so much loss thereby to extract the characteristics of the object very accurately.
Conventionally, the line converter has been generally designed to have such a structure that the widths of signal and ground conductors of the coplanar line portion correspond to the sizes required by a wafer probe head. One end of the converter is connected to one end of the microstrip line so that the signal conductor width is changed smoothly on both sides-. The ground conductor of the coplanar line is thus connected to the ground conductor of the microstrip line via a through conductor such as a through-hole and a via hole.
FIG. 16 shows a top view of the structure of a conventional line converter. A conductor film is applied to almost the entire of the bottom surface of a dielectric substrate 1 having a relative dielectric constant of 9.6 and a thickness of 200 .mu.m to form a ground conductor. Then, the width of the signal conductor 2 of the microstrip line portion and the width of the signal conductor 3 of the coplanar line portion are set to 190 .mu.m and 160 .mu.m, respectively, and the interval between the signal conductor 3 of the coplanar line portion and the ground conductors 4 and 4' is set to 135 .mu.m. The ground conductors 4 and 4' are electrically connected to the ground conductor formed on the bottom surface via 150 .mu.m diameter through-holes 5 and 5' which are through conductors. The structure of each ground conductor of the coplanar line portion is thus formed like a through-hole pad. If the electric characteristics are measured and extracted from those two ground conductors of the same shape formed as described above and placed so as to face each other symmetrically like an object and its mirror image via the microstrip line portion, the frequency characteristics as shown in FIG. 17 are obtained.
In FIG. 17, the lateral axis indicates frequencies in units of GHz, and the ordinate axis indicates transmission coefficients in units of dB used as evaluation indices for the amount of transmitted signals of all the input signals. The characteristic curve indicates the frequency characteristics for transmission coefficients. From this measurement result it is found that the higher the frequency is, the smaller the transmission coefficient is and the more the amount of transmitted signals is reduced.
In addition to such a high-frequency wave measurement substrate composed as described above, there is also another type high-frequency wave measurement substrate disclosed as "Microstrip Line portion Measurement Jig" in Japanese Registered Utility Model Publication JP-Z2 2507797. Unlike the above measurement substrate, this jig is formed by converting the coplanar line and the microstrip line without using any through conductors such as through-holes and via holes. According to JP-Z2 2507797, a measurement jig (measurement substrate) 10 is structured as shown in FIG. 18 (top view) so that the tip of a microstrip line 12 provided on an dielectric substrate 11 which has a ground conductor on its bottom surface is stepped or tapered. Its width is thus matched with the width of a center conductor of a probe head 13 and connected to the center conductor. Then, around the tip of the microstrip line 12 is formed an equivalent ground with a semi-circular or an approximate semi-circular fan-shaped radial stub 14 thereby to correspond to two ground line conductors of a probe head 13. In addition, the radius of a radial stub 14 is decided to be an effective length of about 1/2 wavelength of the lower limit of the measurement frequency.
The utility model has proved that measured data can be reproduced very well with such a configuration of the measurement jig, since no connecting means is used between the ground conductors for connecting the probe head 13 to the measurement jig 10 using an element whose characteristics are varied like the ribbon bonding and the through conductor described above.
It may be said that the principle of the equivalent ground formed with this semi-circular or fan-shaped radial stub 14 is equivalent to a general phenomenon of the radial stub to occur in a high-frequency wave circuit.
In other words, on the basis of IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 36, NO. 7, JULY 1988 "A Coplanar Probe to Microstrip Transition", a reactance value X of a radial stub 15 shaped as shown in FIG. 19 (top view) will be represented in the following expressions, wherein h is a thickness of the substrate on which this radial stub 15 is formed, r.sub.1 and r.sub.2 are inner and outer diameters of the radial stub 15, .theta. is a radial center angle, .epsilon..sub.re is an effective relative dielectric constant in the case where a high frequency wave signal transmits a radial along a radius, .lambda..sub.0 is a free space wavelength of the high frequency wave signal. ##EQU1##
In the above expressions, J.sub.i (x) and N.sub.i (x) are i-order Bessel functions.
According to the principle, the operation of the radial stub in a high-frequency wave goes into an almost perfect reflection state, so that the radial stub can be regarded to be an equivalent ground. Accordingly the radial stub with such an effect is usable as an equivalent ground in a high-frequency wave measurement substrate. The radial stub 14 disclosed in JP-Z2 2507797 uses such effect, and characteristics of a high-frequency wave measurement substrate of the radial stub are extracted.
FIG. 22 is a top view indicating a conventional high-frequency wave measurement substrate which uses a radial stub. The conventional high-frequency wave measurement substrate is formed as a fan-shaped radial stub having inner and outer diameters of 215 .mu.m and 580 .mu.m, respectively, and a center angle of 230.degree. in such a manner that firstly a metallic film as a ground conductor is coated almost all over the bottom surface of an dielectric substrate 21 having a relative dielectric constant of 9.6 and a thickness of 200 .mu.m, then a microstrip line signal conductor 22 as well as coplanar line signal conductors 23 and 23' are formed on the top surface of the substrate, and thereafter coplanar line ground conductors 24 and 24' are formed at distances of 135 .mu.m from the signal conductors 23 and 23'. The electrical characteristics of this high-frequency wave measurement substrate are measured and measurement results obtained are as shown in FIGS. 20 and 21.
In FIG. 20, the lateral axis indicates frequencies in units of GHz and the ordinate axis indicates reflection coefficients in units of dB as evaluation indices for the amount of reflected signals of all the entered signals. In FIG. 20 a characteristic curve S indicates simulation results and a characteristic curve M indicates measured values. In FIG. 21, the lateral axis indicates frequencies in units of GHz and the ordinate axis indicates transmission coefficients in units of dB as evaluation indices for the amount of transmitted signals of all the entered signals. In FIG. 21 a characteristic curve S indicates simulation results and a characteristic curve M indicates measured values. It will be understood from these results that using a radial stub as an equivalent ground is very effective to obtain a high-frequency wave measurement substrate having low loss transmission frequency band characteristics.
In the case of the conventional high-frequency wave measurement substrate as described above, however, when it uses any through conductors such as through-holes and via holes as shown in FIG. 16, the grounds are not stabilized due to the inductance component of those through conductors in a microwave band, and even in a millimeter wave band. Consequently, the continuity of the characteristic impedance is lost, whereby input signals are more reflected and the amount of transmitted signals of high-frequency wave signals is reduced. In addition, the prior art has been confronted with a problem that it is difficult to manufacture such a high-frequency wave measurement substrate very accurately, since it needs processes for processing the through conductors.
Furthermore, when an equivalent ground formed as a semi-circular or fan-shaped radial stub is used as shown in FIG. 18 and FIG. 22, it is required to set the thickness properly for the dielectric substrate. Otherwise, it is difficult to obtain a predetermined effect of the equivalent ground even in a frequency in which the effect is expected as a matter of course. In addition, the high-frequency wave measurement substrate is adversely affected by a high-order mode, whereby the amount of transmitted high-frequency signals is reduced.
Furthermore, when an equivalent ground formed as a semi-circular or fan-shaped radial stub is used as shown in FIG. 18 and FIG. 22, the charge density distribution in the circumferential direction becomes a standing distribution, in which the charge density rises both at the end and at an intermediate point of the semi-circle or fan-shape in the circumferential direction in a frequency in which the length of the semi-circle or the fan-shape in the circumferential direction is equal to an effective value of one wavelength around the center of the semi-circle or the fan-shape in the radial direction. As a result, the equivalent ground generates a resonance. Consequently, the effect of the equivalent ground is hardly obtained around the resonant frequency. The continuity of the characteristic impedance is thus lost and this causes input signals to be reflected more and the transmitted high-frequency wave signals to be reduced more. In addition, when this resonant frequency exists in the low loss transmission frequency band or around the band, the measurable frequency band of the high-frequency wave measurement substrate is narrowed.